Thermionic-cathode for pre-ionization of an extreme ultraviolet (EUV) source supply

ABSTRACT

A source of soft x-rays in an Extreme Ultraviolet (EUV) lithography system may include a pre-ionization unit to pre-ionize a source material, e.g., a Xenon plasma. The pre-ionization unit may be integrated with a discharge unit, and may use Lanthanum Hexaboride (LaB 6 ) as a thermionic emitter material.

BACKGROUND

[0001] The progressive reduction in feature size in integrated circuits(ICs) is driven in part by advances in lithography. ICs may be createdby alternately etching material away from a chip and depositing materialon the chip. Each layer of materials etched from the chip may be definedby a lithographic process in which light shines through a mask, exposinga photosensitive material, e.g., a photoresist.

[0002] The ability to focus the light used in lithography, and hence toproduce increasingly smaller line widths in ICs, may depend on thewavelength of light used. Current techniques may use light having awavelength of about 193 nm. The use of “soft” x-rays (wavelength rangeof λ≈10 to 20 nm) in lithography is being explored to achieve smallerdesired feature sizes. Soft x-ray radiation may also be referred to asextreme ultraviolet (EUV) radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003]FIG. 1 illustrates an Extreme Ultraviolet (EUV) lithography systemaccording to an embodiment.

[0004]FIG. 2 illustrates a gas discharge plasma source according to anembodiment.

[0005]FIG. 3 is a graph describing the relation between life andbrightness of Tungsten and Lanthanum Hexaboride.

[0006]FIG. 4 is a graph describing the relationship between heatingtemperature and electron emission density of thermionic cathodematerials.

DETAILED DESCRIPTION

[0007]FIG. 1 illustrates an Extreme Ultraviolet (EUV) lithography system100 according to an embodiment. EUV lithography is a projectionlithography technique which may use a reduction optical system andillumination in the soft X-ray spectrum (wavelengths in the range ofabout 10 nm to 20 nm). The system 100 may include a source of softX-rays, condenser optics 105, a reticle mask 110, and an optical systemincluding four high precision mirrors (M1-M4) 115-118. The efficiency ofthe lithography system 100 may be reduced due to absorption of EUVlight, which may be absorbed in the atmosphere and by many materials. Toreduce absorption in the system, the lithography process may be carriedout in a vacuum, and a reflective, rather than transmissive, reticlemask 110 may be used.

[0008] The source of soft X-rays may be a compact high-average-power,high-repetition-rate laser 120 which impacts a target material 125 toproduce broad band radiation with significant EUV emission. The targetmaterial may be, for example, a plasma generated from a noble gas, suchas Xenon (Xe). The target material may convert a portion of the laserenergy into EUV radiation with an energy of about 90 to 100 eV.

[0009] The condenser optics 105 may collect EUV light from the sourceand condition the light to uniformly illuminate the mask 110 andproperly fill an entrance pupil of the camera. The condenser 105 mayinclude a series of aspheric mirrors, which collect the radiation andreflect it at a low angle. The mirrors may include multiple layers, suchas alternating molybdenum and silicon beryllium layers to produceconstructive interference in the direction of reflection.

[0010] The radiation from the condenser 105 may be directed to the mask110. The mask may include a multiple-layer reflecting substrate with apatterned, absorbing overlayer. The reflected EUV radiation from themask 110 may carry an IC pattern on the mask 110 to a photoresist layeron a wafer 130 via the optical system including multilayer mirrorsM1-M4. The entire reticle may be exposed onto the wafer 130 bysynchronously scanning the mask and the wafer, e.g., by a step-and-scanexposure.

[0011] As described above, EUV radiation may be generated by impacting aplasma with a laser. FIG. 2 illustrates a gas discharge plasma source200 according to an embodiment. The plasma source may generate a plasma205 from an ionized Xenon gas (or vapor). The plasma may be, e.g., aZ-pinch plasma, a cylindrical plasma which may be established betweentwo electrodes by means of transient electrical discharges. The electriccurrent may heat the plasma and simultaneously produce a Lorentz forcewhich compresses the column.

[0012] The plasma 205 may include a gaseous mixture of neutral species,ions, and electrons. A source of electrons, e.g., a discharge unit 210,may provide electrons as electric discharge initiators which stripelectrons from the Xenon atoms in the gas to produce Xenon ions.

[0013] The discharge source 200 may include a discharge unit 210including an anode 215 and a cathode 220. The plasma 205 may be producedby applying a high voltage pulse across an anode-cathode gap, which maybe filled with gas. The plasma may be imploded by an azimuthal magneticfield produced by an axially flowing discharge current. Duringcompression and stagnation, the kinetic energy may be converted tothermal energy and radiation, and a hot and dense core may be formed atthe center of the discharge unit. The plasma 205 may emit radiation inthe x-ray regime, including soft x-ray, or EUV, radiation. Thisradiation may be filtered for a desired wavelength for use in thelithography system 100.

[0014] The Xenon ions in the plasma may have charges of, e.g., 9+ or14+. To generate the plasma, a number of electrons (e.g., nine tofourteen) may be stripped from Xenon particles in the gas stream. Thisoperation may require a significant amount of energy, and consequently,generate a significant amount of heat. To reduce the amount of energyrequired to produce such ions, the ionization may be performed instages.

[0015] The first ionization stage may be performed by a pre-ionizationunit 225 integrated into the discharge source 210. The pre-ionizationcathode 230 may include a cathode 230 with a thermionic emissionmaterial 235 on its surface. The thermionic emission material, e.g.,Lanthanum Hexaboride (LaB₆), may be used as an electron source whichemits electrons from its surface with the application of heat. Theelectron flux from the surface of the cathode pre-ionizes source gas(e.g., Xe) being transported through or across the cathode surface. Thenumber of electrons generated may be controlled by varying apre-ionization voltage. A pre-ionization of the gas may occur throughcollisions of the cathode emitted electrons with the gas species.

[0016] The current density emitted by the thermionic emission materialmay be described by the Richardson-Dishman equation:

J(T)=A T ² exp (−Φ(/kT)

[0017] where

[0018] A=Pre-exponential constant

[0019] T=temperature

[0020] K=Boltzman's constant

[0021] Φ=surface work function.

[0022] The work function represents the amount of energy an electronneeds to overcome to be emitted from the material's surface. A goodthermionic emitter may be characterized by a combination of a lowsurface work function and a high operating temperature, e.g., have ahigh melting point. Hover, higher melting point materials typically havehigher work functions.

[0023] Lanthanum Hexaboride has a relatively low work function(Φ=2.3-3.4 eV) compared to other high-temperature stable materials, suchas Tungsten (W) (Φ=4.5 eV). The thermionic emission material may besingle crystal Lanthanum Hexaboride (LaB₆) material, and may have acrystallographic orientation of (100), which has a work function ofabout 2.66 eV.

[0024] The plasma may have very energetic ions, e.g., with energiesbetween about 90 eV and 100 eV. Hence, the material used for the cathodeelectrode may be exposed to very high temperatures. Lanthanum Hexaborideis robust with respect to high temperature vacuum environments, as shownin FIG. 3.

[0025] The source and condenser optics 105 may be a closed system. Thecondenser optics may be very sensitive to contamination. For example,one or two monolayers of contamination may reduce reflectivity below anacceptable tolerance level. Debris may be produced in higher workfunction materials due to chunks of material being pulled off or meltingof the material due to high voltage potential at the surface of theemission material. The (100) crystalline Lanthanum Hexaboride materialmay produce less debris in the source due to its lower work function,which enables the material to easily release electrons.

[0026] As shown in FIG. 4, Lanthanum Hexaboride emits electrons of acontrolled number density and energy based on temperature. A feedbackcontrol system may be used to precisely control electron emission fromthe thermionic material. Also, the ability of the monolithic cathode toindependently control supply of pre-ionization electrons may reducedebris from the source insulator and anode.

[0027] A number of embodiments have been described. Nevertheless, itwill be understood that various modifications may be made withoutdeparting from the spirit and scope of the invention. Accordingly, otherembodiments are within the scope of the following claims.

1. A method comprising: pre-ionizing a source material with a thermionicemitter having a work function less than about 3.3 eV; ionizing thesource material to a desired ionization state; and generating a plasmafrom the source material, said plasma emitting an extreme ultravioletradiation.
 2. The method of claim 1, wherein said pre-ionizing thesource material comprises heating a thermionic emission materialincluding Lanthanum Hexaboride (LaB₆).
 3. The method of claim 1, whereinsaid pre-ionizing the source material comprises heating a single-crystalLanthanum Hexaboride material.
 4. The method of claim 1, wherein saidpre-ionizing the source material comprises heating a thermionic emissionmaterial including a single crystal Lanthanum Hexaboride (LaB₆) materialhaving a (100) crystallographic orientation.
 5. The method of claim 1,wherein said pre-ionizing the source material comprises heating athermionic emission material having a work function less than about 2.7eV.
 6. The method of claim 1, wherein said generating a plasma comprisesgenerating a plasma emitting radiation including a wavelength in therange of about 10 nm to about 20 nm.
 7. The method of claim 1, furthercomprising: guiding said extreme ultraviolet radiation to a photoresistmaterial on a substrate; and exposing the photoresist material with atleast a portion of said extreme ultraviolet radiation.
 8. The method ofclaim 1, wherein the source material comprises a noble gas.
 9. A sourceof extreme ultraviolet radiation comprising: a pre-ionization cathodeincluding a thermionic emission material having a work function lessthan about 3.3 eV; an anode; and a pre-ionization voltage supplyoperative to generate a voltage between the pre-ionization cathode andthe anode sufficient to pre-ionize a source material.
 10. The source ofclaim 9, further comprising: a discharge cathode; and a dischargevoltage supply operative to generate a voltage between the anode and thedischarge cathode sufficient to generate a plasma from a pre-ionizedsource material
 11. The source of claim 10, wherein said plasma emitsradiation in the extreme ultraviolet spectrum.
 12. The source of claim10, wherein said plasma emits radiation in a range of about 10 nm toabout 20 nm.
 13. The source of claim 9, wherein the thermionic emissionmaterial comprises Lanthanum Hexaboride.
 14. The source of claim 9,wherein the thermionic emission material comprises a single-crystalmaterial.
 15. The source of claim 14, wherein the thermionic emissionmaterial has a crystallographic orientation of (100).
 16. The source ofclaim 9, wherein the thermionic emission material has a work function ofless than about 2.7 eV.
 17. A system comprising: a source of extremeultraviolet radiation including a pre-ionization module operative topre-ionize a source material, the pre-ionization module including athermionic emitter having a work function less than about 3.3 eV; asubstrate having a surface including a photoresist material; and aplurality of mirrors operative to direct at least a portion of theextreme ultraviolet radiation toward the photoresist material.
 18. Thesystem of claim 17, wherein the thermionic emission material comprisesLanthanum Hexaboride.
 19. The system of claim 17, wherein the thermionicemission material comprises a single-crystal material.
 20. The system ofclaim 19, wherein the thermionic emission material has acrystallographic orientation of (100).
 21. The system of claim 17,wherein the thermionic emission material has a work function of lessthan about 2.7 eV.
 22. The system of claim 17, further comprising afilter operative to filter a wavelength in the range of from about 10 nmto about 20 nm from the extreme ultraviolet radiation.